Electroencephalography (EEG) is a method of monitoring electrical activity of the brain over time, as captured using multiple electrodes placed on the scalp. Information about a person's mental and physical condition can be derived from an analysis of the so-called brain waves observed in EEG signals. One such condition is the transition between a wakeful state and a drowsy state.
In human sleep, there are two main types of sleep—namely rapid-eye-movement (REM) sleep and non-rapid-eye-movement (NREM) sleep. REM sleep is identifiable in an EEG signal by its characteristic low-amplitude, high-frequency waves. NREM sleep is categorized in three distinct stages: N1, N2, and N3; and presents very different EEG wave patterns from REM sleep. For example, N3 NREM sleep exhibits brain waves in an EEG signal characterized by high-amplitude, low-frequency waves. In general, brain waves become slower and more synchronized as a person transitions from stage N1 to N3 sleep.
As a person falls asleep, the activity of their body slows down and their brain waves get slower and bigger. N1 NREM sleep, which is often referred to as drowsy sleep or drowsiness, usually occurs between sleep and wakefulness. This stage is characterized in EEG signals as a transition between so-called alpha waves having a frequency of 8-13 Hz (which is common in the awake state) to so-called theta waves having a frequency of 4-7 Hz. Thus, the transition from a wakeful state to a drowsy state can be easily seen in an EEG signal.